"I just tried the final gathering in XSI2.0, with the same setting(accuracy100, min0.001,max0.01) it seems to be a lot slower than XSI 1.5"
In certain scenes I found v2 to be the same or slightly slower than v1.5. Scenes that use FG and/or GI have always been faster in v2 than v1.5. And not just a small percentage increase, but between 150-400% the speed of v1.5.
A friend of mine created a flying plane scene that uses Final Gathering and HDRI. To render Frame 1 in v1.5 took about 4 minutes. One day in the office, tech support took out the second CPU in his machine for use in another machine - unknowing to my friend. A week later and he upgraded to v2 (still using 1 CPU). Frame 1 of the plane scene took just under 1m40s in v2 and he was very pleased with the results. A couple of days later he discovered the fact that his dual machine didn't have 2 CPU's. Using both CPU's the Frame 1 took less than 50 seconds to render. I was amazed....
Your scenes may need some tweaking in v2.0 to get faster results. The accuracy parameters behave a little differently in v2.0.